Field of the Invention
The present invention and its embodiments relate to the removal of a given chemical species from a liquid. In particular, the present invention and its embodiments relate to the use of a metal-organic framework having particular properties suitable for adsorption of a given chemical species from a liquid stream.
Description of Related Art
Nuclear utilities are challenged with the removal of several impurities that significantly contribute to or drive dose, radioactive waste generation, environmental effluent waste concerns, and materials degradation issues. Analogously, fossil based power generation facilities are challenged by the regulatory discharge requirements for wastewater from flue gas desulfurization and from scrubber, fireside washing, and boiler cleaning operations, as well as by the mandates for groundwater remediation due to coal pile run-off and ash pond leachates. Current technologies (e.g, ion exchange) lack the ability to remove these impurities to the extent needed due to factors associated with the mechanism of capture and competition with other impurities.
Recently developed sequestration media offer organo-metallic ligands decorated on resin backbones (in the locations which would otherwise bear a cation exchange group) that are significantly improved removal media for analytes that are cations like cobalt. Unfortunately, such ligands cannot accommodate the larger geometry of oxy-anions of species like selenium that are found in the subject water streams. For example, ion exchange and adsorption technologies are typically used to capture chemical impurities in water streams. However, these technologies are subject to several significant drawbacks. They are non-specific (i.e., will capture many different species to some degree), subject to competition (i.e., higher concentration species will dominate), and are reversible (i.e., captured species will be released given changes in water conditions).
Nonetheless, removal of selenium from water streams is of particular interest. Selenium is a naturally occurring element that is essential, in low concentrations, for human health. Of all essential elements however, selenium has the most confining range between dietary deficiency (<40 μg/day) and toxicity (>400 μg/day). Selenium enters our waterways through a number of different sources such as agricultural runoff, mining, industrial production, and via flue gas desulfurization processes. As a consequence of the narrow range between deficiency and toxicity, it is very important to monitor and control the amount of bioavailable selenium in our drinking water. The U.S. Environmental Protection Agency recognizes the dangers of selenium and has mandated the maximum acceptable level for selenium in drinking water to 50 ppb. However, in more recent proposals, regulatory plans to reduce selenium discharge requirements to 14 ppb and then as low as 10 ppb, making the present operation of many flue gas desulfurization wastewater cleanup facilities incapable of achieving such purity without methods beyond typical ion exchange or adsorption engineering unit operations.
Selenium can occur in both organic and inorganic forms, but the high solubility and hence bioavailabilty of inorganic species such as selenite (SeO32−) and selenate (SeO42−) makes these anions the primary focus of remediation techniques. Many techniques have been explored for the removal of selenite and selenate from water including the use of vertical flow wetlands and bioreactors, but high start-up costs and size requirements have limited the application of these techniques. An alternative approach that has been investigated involves using an adsorbing media to soak up and remove unwanted inorganic selenium. Iron oxides (hematite, goethite, and ferrihydrite) have been studied extensively as potential adsorbents for selenite and selenate in aqueous solutions. These iron-based materials have very low surface areas, meaning that a lot of the material is wasted due to the lack of available adsorption sites. Iron oxides also tend to be effective for selenite removal due to the formation of inner-sphere complexes between the selenite anion and iron oxide surface while selenate removal is not as sufficient because only weak, outer-sphere interactions occur.
Therefore, a novel technology is needed that effectively and efficiently removes specific impurities from water and other liquid streams in the presence of other competing species, and in a manner that specifically targets capture of that impurity—holding it with a much higher binding energy. In particular, a different type of structural media is required to specifically address removal of low levels of particular species, such as aqueous oxy-anions of selenium, with high enough binding energy to maintain near irreversible uptake as analyte concentrations are lowered while competitor concentrations are simultaneously raised.